1. Field of the Invention
The present invention relates to voltage sensing and measuring, and more specifically, to an apparatus for sensing and measuring voltage with greater precision. The apparatus uses an isolation circuit to isolate the signal provided by a voltage source, such as a lithium-ion cell, from the remainder of the measuring circuit to ensure precision measurement even at high voltage levels.
2. Description of the Related Art
Lithium-ion cells are being used in greater numbers of space applications, such as to power satellite electronics and control systems. Accurate sensing of the state of the charge of these cells is very critical for lowering system mass and increasing battery life. Typically, the state of charge of a lithium-ion battery can be derived from the voltage of each cell. Therefore, voltage telemetry precision is of utmost importance. The more components used in the circuit, means more tolerance that must be accounted for in a worst case accuracy analysis.
U.S. Pat. No. 6,211,650 to Mumaw describes a method of limiting the charging voltage applied to an individual cell of a plurality of cells making up a battery in which the actual voltage of an individual cell has been sensed. However, this patent does not suggest a use for precision measurements of voltage by means of an isolation circuit. In addition, this patent does not suggest an attempt to reduce the measurement error through a minimal number of elements.
U.S. Pat. 6,157,171 to Smith describes a method for monitoring the voltage of a rechargeable battery using an integrated circuit. This patent does not suggest the use of a circuit, such as a transformer, to isolate the cell from the rest of the circuit. Furthermore, there is no suggestion that an emphasis has been placed on precision and accuracy of the measurements.
U.S. Pat. 6,077,624 to Mitchell describes the general operation of a lithium-ion battery. The patent suggests an improved method to improve the thermal stability of a lithium-ion cell. While the detailed components and operation of this type of cell are described in the patent, there is no description of a voltage sensing device that employs an isolation circuit.
A continuing need exists for improved circuitry for measuring voltage levels on a precision and accuracy basis.
An object of the present invention is to provide a voltage telemetry circuit which uses a xe2x80x9cminimalxe2x80x9d circuit approach to sampling cell voltage.
Another object of the present invention is to provide a voltage telemetry circuit that minimizes errors related to component tolerances, and allows the voltage to be determined repeatedly and predictably.
A further object of the invention is to provide a method for isolating the input signal to the isolation circuit from the measurement circuitry.
The present invention meets these objectives by providing an isolation circuit that isolates a cell from measurement circuitry. Preferably, the invention provides an output voltage that is the same as the input voltage applied to the isolation circuit. An approximation of an ideal isolation circuit provides an output that differs from the input of the isolation circuit by a predictable amount. One such circuit is a pulse transformer. The output of a pulse transformer includes a predictable and repeatable offset from the input.
Other and further objects of the present invention will be apparent from the following description and claims and are illustrated in the accompanying drawings, which by way of illustration, show preferred embodiments of the present invention. Other embodiments of the invention embodying the same or equivalent principles may be used and structural changes may be made as desired by those skilled in art without departing from the present invention and the purview of the appended claims.